Quite a lot not sure personally but it was in development for a like ages and I do mean ages. Here's the timeline

- Blizzard North begins development of there version of III
- They ''failed'' apparently and the idea was scrapped all together you can see some of the old screen shots from google too lazy to look at it
- A new team which was Jay Wilson was set into place while North became Runic games and split off completely
- Alpha was created the version you can see is on youtube a very very early concept of it which actually does look better but they said fuck it
- Alpha was scrapped and they decided to revamp the system again
- Beta comes in for the employees
- a small ''beta'' was for customers who were chosen aka up to Skeleton King

They kept pouring in resources then scrapping it again and again so I can imagine a shit ton of money because they kept fucking up the time line within those events are from 2004s to 2011

It does not work like that.
There is no "development cost" for big projects. The developemnt is split between departments and everyone is playing their own small(or big) role.
There are a lot of expenses not directly related to development, but essential for the game. Building a new datacenter in china or paying $50M yearly bonus to one of the CEO would be a good example of such expenses.
Another good example of such expenses will be D3v1. As you know, D3 that was released is actually d3v2. But Blizzard spend a lot of time(at least 3-5 years) and money for developing d3v1 as well. And it is included in development cost.

If you are curios, blizzard is not making a lot of money.
WoW is paying blizzards bills and without wow there would be no D3 or SC2.
WoW is giving blizzard a HUGE yearly revenue, at least 1 billion every year just for subscription. That's like releasing d3 every year, but without significant development expenses.

But net income is probably 10% of this number. Most of the money they earn they spend for feeding the 5000-employees corporate monster they created.

What is the actuall price of the development itself? Not very big I guess, like everywhere in IT it's really just developers and designers salary. And apart from several CEOs, developers and designers in blizzard do not get THAT much money.
TL2 and PoE were developed with under 20M budget. a

But for D3... no one will be able to give you even a rough estimation.

Quite a lot not sure personally but it was in development for a like ages and I do mean ages. Here's the timeline

Around 12 years of development and at least one complete restart.

Diablo III's development began in 2000, and it was going to be an MMO, because they wanted a 'big overworld and giant shared community' to provide a cool, chaotic experience with the emerging MMO technologies of the time. They were not concerned at all with 'what the guys down south' aka World of Warcraft were doing, because WoW was going to be different enough from what Blizzard North was doing, and the Warcraft and Diablo communities were historically different, so they believed they could have had a Diablo MMO and a Warcraft MMO at the same time.

In 2003, the 'Big Four' founders of Blizzard North/Diablo (Max Schaefer, Erich Schaefer, Bill Roper, and David Brevik) left after distrust over Vivendi and their financial futures or something like that. Work was underway on the game by that point, and the High Heavens were not part of it.

They were actually working on two projects at the time, the other involved 'four races' including dragon-y and lizard people. Work on that secret project was scrapped and all hands at Blizzard North were moved to Diablo III, whose development was continued by the Blizzard North remnants until 2005. This is the 'other D3' we have seen screenshots and so forth of.

Diablo III was likely re-envisioned after they lost a lot of their talent in 2003, because according to Max Schaefer when he was asked about the 2005 version: "That’s not what we were making at the time."
Bill Roper also made a similar comment, "Nothing that I've seen looks like anything I ever did with the game, so I would imagine they got rid of the vast majority of what I did when I was there."
Mr. Schaefer also recently confirmed it was apparently still going the MMO route up to 2005, and was only changed when it was brought to Blizzard Irvine, due to "different design priorities and goals than we did."

Not too much is known about this 2005 D3 except that it appeared to have the general storyline and locations of the current D3, including the High Heavens.

Rumors about the game, allegedly from employees/insider sources in late-2005, 2006, include claims that there was going to be enhanced multiplayer aspects, guild housing, mounts, and perhaps most interesting, a 'two-faction' system of Heaven versus Hell that included 'light and dark' versions of every item in the game. These alternate items were allegedly one of the 'problems with the original version of the game' and didn't do anything besides alter the visuals.

Why did this version get scrapped? Jay Wilson said that it felt a little like Diablo 2.5, that it was maybe too reverent of Diablo 2 and didn’t break enough new ground.

From 2006-2012 the D3 Project continued at Blizzard, and if you look at the footage and screenshots from WWI and subsequent BlizzCons, you can see the game stayed mostly the same, but became less 'ground breaking' as the years went by. Notably, a lot of the features shown in 2008 never made it in like being able to climb ladders, obstacles to clear with class abilities like Leap, in-game cutscenes for dialogue, even the way the dialogue was displayed, etc. It also went through a lot of iterations of the skills, stats, runes, and other mechanics.

12 million copies is a lot for most video companies, but it is pocket change for Blizzard (WoW).

"Vivendi sold shares back to Blizzard for $8 billion" is real a bad news for Blizzard.
Now Blizzard is in $4 billion debt.

I don't think WoW+CoD+Diablo can make $4 billion any more.
Any WoW lose will bankrupt this company.

Do your home work first dude.

ATVI is valued at 19 billion dollars.

Its expected revenue for this year is 4.25-4.3 billion dollars.

ATVI stocks are now rated at +25% since the deal.

The short term debt of 1.2 billion is spread over 3 years and the net profits are expected around 1 billion per year...ATVI had a war chest of 4.3 billion dollars that would have vaporated if Vivendi had forced ATVI it to be used to fill the bottomless pit of over 10 billion dollars losses of the French media group.

Since ATVI is now complete free in operations and no longer is controlled by a French media group, the situation of Blizzard has never been better

WoW is still good for around 1 billion dollars per year, just like COD,and all the other games present and upcoming: Hearthstone, Bungy's new Destiny, Blizard's Dota.

As a result the yearly stocks overview went from 11 dollars to 18 dollars.

As an interesting side note: WoW Gaap revenue of the last quarter was bigger than last years quarter and hardly was down on non gaap revenue.

So in the end I still have my stocks as the company has all its jokers now in proper hands.

If ANY of the upcoming games Hearthstone, Blizzard's Dota or Destiny are half the successes of Diablo 3, the group will certainly grow further because of the new console launches, where the company has its bigger shares.

The only critical point made by some stock brokers is the reluctance of Bobby Kotick to invest in tablet games. But Hearthstone is a giant step towards this as it will be launched on i Pad worldwide with already a complicated China launch included.

If anything, Heartstone which combines free to play, with WoW Lore, multi platforms, Collectable card games mechanics and world wide launch is a sure hit, because everyone will try the first free to play game of Blizzard.

And on top HS beats digital MTG over on line mechanics already (MTG has way too complicated mechanics for on line play to attract the masses).